You can use a UIImageView and place it on the background of a uiview so you can layout native buttons and objects to best imitate the graphics they sent you using native controls or you can use other helpers online such as these.
Honestly, IMO it's generally better to use their graphics as a template then recreate what they are wanting to achieve using native controls to adhere to the HIG Apple provides. Giving the user a nice at home feeling while also giving the client the look they are wanting to achieve is the general idea to a successful app. Each OS has it's own "look and feel" that the users come to expect.